Resprouting as a key functional trait in woody plants: challenges to developing new organizing principles
Most global biomes are shaped by disturbances, such as fire or herbivory, that damage or kill the aboveground biomass of plants. In many of these biomes, however, the damaged plants do not die; rather, they persist through sprouting. In disturbance-prone environments, resprouting from meristems stabilizes plant populations where disturbance may cause demographic bottlenecks. The advantage of resprouting is that it confers persistence under disturbance; however, sprouting has disadvantages such as potentially reducing sexual reproduction and limiting gene flow (Bond & Midgley, 2001; Lamont & Wiens, 2003). Understanding resprouting is critically important for understanding long-term vegetation dynamics, extinction risks, carbon balance and woody plant management. A Vegetation Function Working Group (Working Group 67; http://www.vegfunction.net/wg/67/67_Sprouting.htm) was established in 2009 to identify challenges encountered in developing coherent models of the functional role of resprouting in fire-prone environments, with an emphasis on savanna and Mediterranean biomes. Fire is the most pervasive disturbance and is integral to the evolutionary ecology of these biomes. This broad disciplinary group met at the International Ecological Conference (INTECOL) in Brisbane in September 2009 and reconvened in July 2010 at The University of New England, Armidale, Australia, to review progress. In this report we highlight major challenges encountered in developing unifying models of the functional role of resprouting.
Ecology of plant resprouting: populations to community responses in fire-prone ecosystems
Resprouting has emerged as a key functional trait in plant ecology over the past decade with more than 400 papers published since 2000 (Web of Science). This special issue of Plant Ecology brings together a set of papers that advance our understanding of this functional trait, in the quest for developing a better conceptual framework for predicting community response to disturbance. This special issue highlights current research on all aspects of the effects of fire and other disturbances on plant resprouting behaviour and the importance of the persistence niche in structuring plant communities.
Fire regime, soil fertility and growth form interact to shape fire and growth traits in two co-occurring 'Banksia' species
Inter- and intra-specific comparisons were made between several co-occurring populations of two 'Banksia' species growing in Eastern Australian dry sclerophyll open woodland that experiences a grass-fueled surface-fire regime. The two species differ in life-history from a short basal resprouter (B. 'Neoanglica') to a tall fire-survivor (B. 'integrifolia'). Growth (LMA = leaf mass per unit mass area), persistence (bark thickness) and recruitment (serotiny) traits were determined across independent gradients in soil fertility and fire frequency. Trait correlations for the two species showed distinct patterns, from each other and across environments, with the resprouter having higher LMA and infructescence retention but lower bark thickness. However, there were no consistent intraspecific patterns with variation in fire and soil nutrients. LMA only varied in B. 'integrifolia' with soil fertility, and fire frequency had no effect on either species. Relative bark thickness varied with plant size but not among sites. There was a trend of increasing pyriscence with fire in B. 'neoanglica' and with lower nutrients soils but not in B. 'integrifolia'. Clearly the two species respond differently to variability in nutrients and fire. Nevertheless, growth form appears to be the strongest determinant of both growth (LMA) and fire traits (thicker bark; pyriscence) in these co-existing 'Banksia' species.
A burning issue: community stability and alternative stable states in relation to fire
Fire regimes have long been thought to drive plant community change in biomes by altering feedbacks that maintain "stable" community assemblages (see Jackson, 1968; Mutch, 1970). The occurrence of contrasting vegetation types in otherwise comparable environments, where flammable communities are juxtaposed with those that rarely burn, is often explained by alternative stable state (ASS) theory, where shifts in equilibrium are triggered by catastrophic fire (Petraitis & Latham, 1999; Scheffer & Carpenter, 2003). When high-intensity fires burn into less flammable communities, compositional change is thought to occur because, firstly, an ecological threshold is reached beyond which the disturbance is large enough to remove species that perpetuate the exclusion of fire, secondly, space is then opened up for colonization by more flammable species and finally self-reinforcing pyrogenic dominance is achieved (Figure 5.1). Alternative stable states are often invoked in fire-prone regions and climates where there are sharp boundaries between communities (Figure 5.2). One of the most cited examples of alternative stable states is the fire-triggered transformation of rainforest to more flammable assemblages (Jackson, 1968; Webb, 1968; Bowman, 2000; Beckage et al., 2009; Hoffmann et al., 2009; Warman & Moles, 2009). Other examples of ASS include the conversion of boreal forest to deciduous forest (Johnstone et al., 2010), tropical savanna to grassland (Hoffmann & Jackson, 2000) and the replacement of arid shrublands by more flammable grassland (Nicholas et al., 2011). Fluctuations in community composition are the norm for most plant communities and in this sense they are "meta-stable" at Holocene timescales even under the effects of severe disturbance. In contrast, transformation in structure and complete floristic turnover at decadal timescales are those associated with ASS.
Costs and benefits of relative bark thickness in relation to fire damage: a savanna/forest contrast
1. In fire-prone ecosystems, bark protects the stem bud bank from fire. Absolute bark thickness is a good indicator of this protective function, but it depends on stem size as well as inherent differences between species. Relative bark thickness (i.e. relative to stem diameter) takes the latter into account. We argue that relative bark thickness is an important functional trait offering insights to the evolution of species persistence in fire-prone habitats. 2. During growth ontogeny different species can acquire absolutely thick bark through having: (i) relatively thick bark (i.e. an early commitment to thick bark) or (ii) relatively thin bark but fast stem diameter growth rates. We test the hypothesis that the most effective way of protecting tree stems from frequent fire is by having relatively thick-barked small stems. We predict that species with higher relative bark thickness are more common in fire-prone habitats. In habitats with long fire-free intervals such as rainforest, delayed investment in bark thickness results in thin bark. 3. We examined the relative bark thickness of woody congeners from Australian non-fire-prone forest and fire-prone savanna and in other tree-dominated systems world-wide. We determined the relative cost of acquiring absolute bark thickness of 0.5 cm for different rates of bark allocation. The insulating benefits of bark were considered a linear function of bark thickness. 4. Synthesis. We suggest that relatively thick bark minimizes the costs of acquiring absolutely thick bark, and it confers greater protection to smaller stems. The cost of acquiring thick bark prevents small trees from merely accumulating bark as a consequence of fast height or stem diameter growth. Accordingly, our field survey indicated that forest species had relatively thin bark and acquired thick bark only as a consequence of very large size, while fire-prone savanna species had relatively thick-barked small stems. Based on this, relative bark thickness appears to be a good predictor of local fire regimes and is a useful plant functional trait.